It is known to provide an alignment layer structure by forming a polyimide layer on a substrate followed by rubbing or buffing. Whilst such a procedure provides a good way of achieving unidirectional alignment, patterned alignment by rubbing relies on rubbing only certain areas of the polymer surface through a mask interposed between the rubbing cloth and the polymer surface. The mask is then replaced with a second mask exposing different areas of the surface and rubbing is repeated in a different direction. These methods are difficult to implement on a manufacturing scale and are only partially successful.
EP-A-0689084 and U.S. Pat. No. 5,602,661 disclose a patterned alignment layer structure which involves exposing through masking different parts of a photo-orientable polymer network (PPN) layer to curing radiation which is polarised in different directions. The direction of polarisation of the radiation which cures the PPN determines the alignment in the exposed parts of the layer structure. The use of radiation polarised in selected directions to control alignment direction is also disclosed by M. Shadt et al, Nature, Vol 391, 1996 pages 212 to 215; M. Shadt et al, Japan J. Appl. Phys, Vol 31 (1992) pages 2155-2164, and W. M. Gibbons et al, Nature, 351, 49 (1991). However, all of these disclosures require a special polymerisable composition for the PPN layer and multiple exposures of the surface of the alignment layer through photo masks with the polarisation state of the curing radiation being altered between exposures.
U.S. Pat. No. 5,602,661 mentions the possibility of incorporating chiral molecules in the PPN layer. However, the purpose of this is to provide different optical properties in the layer, such as colour filtering.
It is an object of the present invention to provide a liquid crystal device having a patterned alignment layer structure which can be produced in a relatively simple and convenient way without the need to effect multiple rubbing steps or to use a plurality of photo masking and polarised light exposure steps.
According to a first aspect of the present invention, there is provided a liquid crystal device comprising a substrate; a patterned alignment layer structure on the substrate; and a liquid crystal layer having a surface in contact with the patterned alignment layer structure, the patterned alignment layer structure having a plurality of alignment regions with different alignment directions, characterised in that the patterned alignment layer structure comprises a first alignment layer having a first alignment direction and a second alignment layer disposed over the first alignment layer and including liquid crystal monomers (hereinafter called "reactive mesogens") which are twisted and which have been cured.
According to a second aspect of the present invention, there is provided a method of producing a liquid crystal device, comprising the steps of providing, on a substrate, a patterned alignment layer structure having a plurality of alignment regions with different alignment directions, and providing a liquid crystal layer on an exposed surface of the patterned alignment layer structure, wherein of the patterned alignment layer structure is formed by providing a first alignment layer having a first alignment direction on the substrate, providing a layer of a curable reactive mesogen composition having a twist structure, on the first alignment layer, and effecting selective regional curing of such composition so as to fix the twist therein.
The plurality of alignment regions preferably include first alignment regions and second alignment regions.
In a first embodiment, the first alignment regions are defined by the first alignment layer, and the second alignment regions are defined by at least some of the cured and twisted reactive mesogens of the second alignment layer, the second alignment layer being patterned so as to expose the first alignment regions defined by the first alignment layer.
In such first embodiment, it is possible to align the first alignment layer simply by unidirectionally rubbing the whole of the first alignment layer. The second alignment layer can then be formed by providing a layer of a curable reactive mesogen composition over the unidirectionally rubbed first alignment layer followed by masking, selective curing of the reactive mesogens to define the second alignment regions, and subsequently removing the uncured regions so as to expose the first alignment regions in the underlying first alignment layer.
In a second embodiment, a curable reactive mesogen composition is used which exhibits a different twist angle at different temperatures. Thus, by selectively curing different regions of the reactive mesogen composition at different temperatures (e.g. by use of non-heating polymerising radiation such as UV radiation), it is possible to fix different twist angles in different regions of the second alignment layer. Thus, in this embodiment, it is not essential to leave uncured regions which are subsequently removed so as to expose parts of the underlying first alignment layer, although this may be done if required. By this technique it will be understood that, by appropriately repeating masking and curing at different temperatures, it is possible to produce any desired number of different alignment regions providing different alignment directions for the overlying liquid crystal layer.
In order to provide the required twisted structure in the second alignment layer, it is convenient to employ a chiral dopant in the second alignment layer since the amount of dopant included can be used to control the degree of twist which occurs in the second alignment layer, thereby controlling the azimuthal alignment of the second alignment direction relative to the first alignment direction. However, it is alternatively possible to use reactive mesogens which are themselves chiral.
In the case where a chiral dopant is included, the concentration percentage, C, of a chiral dopant to be included in a liquid crystal layer of thickness, d, in order to induce a "twist-off" angle .phi..sup.o can be determined by the formula: EQU C=(.phi..times.100)/(360.times.d.times.T.sub.p)
where T.sub.p is a constant (with dimensions of length.sup.-1) known as the `twisting power` of the chiral dopant.
Examples of suitable chiral dopants are CB15 (Merck, T.sub.p =32 .mu.m.sup.-1), R1011 and S1011 which are respectively right- and left-twisting dopants (Merck, T.sub.p =7.5 .mu.m.sup.-1).
In the second embodiment described above, use is made of a curable reactive mesogen composition which exhibits a different twist angle at different temperatures. Chiral dopants exist whose twisting power is a strong function of temperature, some even showing twist inversion (i.e. a change in handedness) with temperature. An example of such a material is 18,19,21,27-tetranorcholesteryl anisoate, see H. Stegemeyer et al, Z. Naturforsch., 44a, 1127-1130, (1989).
In a convenient embodiment, the first alignment layer is a unidirectional alignment layer and may be provided by a rubbed polymer layer, e.g. a rubbed polyimide layer.
In one convenient embodiment, the liquid crystal device comprises a further substrate and a further alignment layer structure disposed in contact with an opposite surface of the liquid crystal layer. Such further alignment layer structure may be a patterned alignment layer structure, e.g. of the type defined above, or it may be a unidirectional alignment layer structure produced, for example, by unidirectional rubbing of a single alignment layer.
It will therefore be appreciated that the present invention can be realised using only a single rubbing operation and a single masking and curing operation to produce the patterned alignment layer structure. It will further be appreciated that there is no need to use polarised light and that the mutual inclination of the first and second alignment directions can be controlled relatively simply by controlling the amount of chiral dopant used in the reactive mesogen composition.
In one convenient embodiment, the alignment directions are mutually azimuthally orientated at 90.degree..
In another convenient embodiment, the first and second alignment directions are mutually azimuthally orientated at 180.degree..
In a particularly convenient embodiment, the first alignment regions alternate with respective second alignment regions.
In a preferred embodiment, selective curing of the layer of twisted reactive mesogens is effected by masking and photopolymerising.
There are no particular restrictions on the type of reactive mesogen used (typical examples are acrylates and vinyl ethers), or on any chiral dopant employed (which may be right or left handed).
There are no particular restrictions on the type of liquid crystal (LC) layer deposited on the patterned alignment layer structure. The LC may be nematic or smectic (e.g. the ferroelectric smectic-C* phase). The LC may itself contain chiral dopants, as in the well known STN and TN displays. There are no particular restrictions on the type of device in which the patterned (i.e. multi-domain) alignment layer structure is employed.